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People Magazinehas reported how Michael's 2010 cancer battle and Catherine's own struggle with bipolar II disorder created a strain on the pair.

'The stress has taken a toll on their marriage,' a friend told the publication.

Deep in thought: Michael Douglas and Catherine Zeta Jones were to separate after 13 years to 'evaluate and work on their marriage'

Separation: The pair haven't been seen together since April

They are said to have decided to spend some time alone after Michael returned from this year's Cannes Film Festival in France in May.

While in Italy last week, Michael was seen playing golf on his own - a hobby he once enjoyed with second wife Catherine.

Michael is said to be living in Manhattan while Catherine is staying at their $4m home in New York State with their children Dylan, 13, and Carys, ten.

The pair have not been photographed by each others side since April, when they attended a gala event at the Lincoln Center in Manhattan.

In February it was claimed by The Enquirer that the pair were to separate in a 'hush hush' divorce. (They denied the claims at the time.)

The Enquirer alleged that the marriage 'buckled under the pressure of dealing with Michael's cancer battle, his son Cameron's incarceration on drugs charges and Catherine's struggle with bipolar II disorder.'

More recently, Star Magazine reported that Catherine was set to begin a '$300m divorce battle' against Michael.

However, in an interview last month with the Daily Mirror, Michael hinted that the couple's struggles had only served to strengthen their bond.

Taking stock: The couple first tied the knot back in 2000 at the Plaza Hotel, however their marriage had recently run into problems.

Looking ahead: Michael appeared to be spending some alone time on aboard the yacht, as a female member of staff passed by in the background

'True love takes work. Love is an extraordinary feeling that comes from the bottom of your soul, but has to be nurtured, you know. The thing I've learned about getting older is you can't take love for granted,' he said.

'You protect it, nurture it and it grows and after one's initial, physical, emotional aspects, it becomes deeper,' added Michael.

On Friday, Michael is expected to attend the Deauville Film Festival in France; poignantly, it is where he first met Catherine in 1998 and famously introduced himself by saying: 'I want to father your children.'

Relaxing: Michael was left to his own devices aboard the yacht as his friend appeared in the background

Taking a break: Michael eventually appeared to smile as re-joined the group on the luxury sailboat

Together apart: Michael and Catherine have not been photographed by each other's side since April, when they attended a gala event at the Lincold Center in Manhattan